Did some of the Covenant on Halo escape? And is that the key to how they found earth?

Recently it occurred to me that it is possible that some of the Covenant on Halo could have escaped alive. We know that the Longswords are very fast - they must be or Cortana and the Master Chief wouldn't have got far enough away from Halo before it's destruction. However, in the intro sequence of the first level Cortana specifically states that the Covenant ships are faster than human ones. Of course it is possible that Cortana did not mean that all Covenant ships are faster than all human ones but even so it seems possible that at least a few of the them could have escaped alive.

This brings up another point. We know that in Halo 2 the Covenant have found and are attacking earth. The Covenant in that dropship were probably on the Pillar of Autumn before they took off and may have examined the ship's systems, as 343 Guilty Spark did. It is possible that they did this while Cortana was in there (when she and the Chief visit the bridge he puts her back into the POA's systems for a short time) and learned the location of earth (Cortana stores this information - this is why Captain Keyes gave her to the Chief to protect). If they did this and then escaped they could have then informed the rest of the Covenant and then they would have launched an invasion of earth, which would explain how they found it.

Well, they didn't leave on the Pillar of Autumnm or the Truth and Reconciliation, that much is clear. Cortana seems to indicate that no one else was present in the Threshold system after the ring's destruction, and we know that the "[Covenant] leadership ordered all ships to abandon Halo when they found the Flood", but one would assume that at least a single ship would have needed to remain to launch the strike teams from...